For jumbo borrowers, though, higher standards aren't the biggest problem: Rates on those loans averaged 6.8% at the end of last week, according to HSH, meaning the spread between conventional and jumbo rates is nearly a full percentage point -- four times the typical gap.

"This is a way of saying, 'We are going to also foster low mortgage rates, and if the market isn't interested in buying them, well, we will buy them,' " said Keith Gumbinger, vice president at financial publisher HSH Associates.